The scope of this project is the study of infections of mink with the Aleutian disease of mink parvovirus (ADV). In the past year we have begun the analysis of ADV infections utilizing strand- specific in situ molecular hybridization. The rationale is based on the fact that replicative forms of ADV DNA (RFs) contain "+" sense nucleic acid strands, whereas virion DNA is largely "-" in sense. Thus, probes that are "-" in sense will preferentially define sites of replication, whereas "+" sense probes will in addition delineate sites where virus is being sequestered. We have applied these probes in in situ hybridization to date on ADV infected tissue culture cells in vitro and on sections of newborn mink with ADV interstitial pneumonitis. Throughout permissive in vitro infection, duplex RFs were localized to the cell nuclei. Virion DNA and capsid proteins were found in the nuclei early in infection, but were later translocated to the cytoplasm. The autoradiographic grains were clearly localized over the infected cells, and the number of cells positive for ADV antigen was the same as that positive for viral DNA, suggesting that in this permissive system, all cells containing viral DNA also expressed viral proteins. In ADV infected neonatal mink, cells replicating ADV were found primarily in the lung. RFs and ADV antigens were found predominantly in the surfactant producing alveolar type II cells, and the pattern was similar to that observed for the tissue culture cells. This observation suggested that the neonatal pneumonitis results from a direct cytopathic infection by ADV of the type II cells, with subsequent depletion of pulmonary surfactant. Viral DNA was also found in other organs, but the strand specific probes made it possible to show that most of this DNA represented virus that was being sequestered in phagocytic elements of the lymphoreticular system. Virion DNA, ADV antigen and mink IgG were found in the histologically normal glomeruli of these infected neonates, suggesting an early form of immune complex nephritis in which intact virions participated.